Container comprising biaxially oriented polystyrene composition and rubber particles

ABSTRACT

The present invention provides biaxially oriented polystyrene composition film and container made of such biaxially oriented film. The films are useful in that they are clear and have improved room and low temperature impact strength. The films include from about 0.1 to about 1.4 weight percent rubber particles, based upon the total weight of the composition. The average diameters of the rubber particles are from about 0.1 micron to about 10 microns.

This application is a division of application Ser. No. 08/294,655, filedAug. 23, 1994, now U.S. Pat. No. 5,643,664.

FIELD OF THE INVENTION

The present invention relates to biaxially oriented polystyrenecompositions which axe clear and have improved low temperature impactstrength. The invention relates as well to a method of using biaxiallyoriented polystyrene compositions.

BACKGROUND

Polymeric materials are used widely to package and transport a widevariety of consumer goods, including perishable items which must betransported and sold at refrigerator or freezer temperatures.Commercially popular polymeric materials include polyethylene,styrene/butadiene copolymer and polystyrene.

At refrigerator or freezer temperatures, pure polystyrene isunsatisfactory for transporting perishable goods because it is brittleand fails during transportation. A known method of increasing the lowtemperature impact strength of polystyrene is the addition of rubber tothe polystyrene composition. Typically, general purpose polystyrene(GPPS) is blended with high impact polystyrene (HIPS), which usuallycontains around 7% rubber particles by weight, to form a GPPS/HIPSadmixture.

The addition of rubber to a polystyrene composition results in increasedimpact strength but increases the opacity of the polystyrene. Clearcontainers for packaging goods are desirable because it allows thecustomer to see the goods before purchasing. Typically, the percentageof HIPS in polystyrene cannot exceed 1% (0.07% rubber content) becausepolystyrene compositions with more than 1% HIPS show improved impactresistance but are too hazy for commercial applications. A polystyrenecomposition comprising 99% GPPS and 1% HIPS is relatively clear but doesnot have a sufficiently high low temperature impact strength andtherefore is not suitable for use at low temperatures. There is a needfor a polystyrene composition comprising more than 1% HIPS (0.07% rubbercontent) which provides higher impact strength at room and lowtemperatures and is clear. Polystyrene compositions are typical ofpolymeric materials in that clarity and strength can be improved byorienting the polystyrene molecules. Orientation of polystyrenecompositions is accomplished by stretching the GPPS/HIPS admixture.Polymeric compositions can be stretched biaxially. The two axialdirections are typically perpendicular to each other. Biaxial stretchingimproves the clarity of polymeric compositions and increases the impactstrength. Biaxial stretching of polystyrene compositions is known (Ayreset al, U.S. Pat. No. 3,995,763). Ayres, however, provides an opaquecomposition. Furthermore, present commercial biaxially orientedpolystyrene compositions are unsatisfactory because of poor impactstrength. Thus, there is a need for a clear biaxially stretchedpolystyrene composition with improved impact strength.

SUMMARY OF THE INVENTION

In one aspect the present invention provides a biaxially stretchedpolystyrene composition having an haze of less than about 10%. Between-30° F. and ambient temperatures, the polystyrene composition has afalling dart impact strength of between about 10 lbs to about 200 lbs.The polystyrene composition includes rubber particles of about 0.1 toabout 1.4 weight percent based on the total weight of the polystyrenecomposition. The average diameters of the rubber particles included inthe polystyrene composition are between 0.1 micron and 10 microns.

In another aspect, a container for transporting and storing perishableitems at freezer temperatures is disclosed. The container is made of abiaxially oriented polystyrene composition having a haze of less thanabout 10%. The container is useful for transporting and storingperishable items because its impact strength at freezer temperatures isbetween about 10 lbs to about 200 lbs. The containers as taught by thepresent invention include rubber particles with an average diameter ofbetween 0.1 micron to about 10 microns. The total rubber content of acontainer by weight is between about 0.1 to about 1.4 percent based onthe total weight of the polystyrene composition.

In a third aspect, the present invention provides a polystyrenecomposition prepared by admixing general purpose polystyrene and highimpact polystyrene and biaxially stretching the admixture to biaxiallyorient the composition. The polystyrene composition prepared by theprocess as described is clear and has less than about 10% haze. Thepolystyrene composition further includes about 0.1 to about 1.4 weightpercent of rubber particles with average particle diameters of around0.1 micron to 10 microns.

DETAILED DESCRIPTION OF THE INVENTION

A biaxially oriented polystyrene composition with an haze of less thanabout 10% and impact strength of about 10 lbs to about 200 lbs attemperatures between -30° F. and ambient temperatures is provided by thepresent invention.

As used herein, haze is defined as the amount of visible light scatteredor absorbed by a biaxially oriented polystyrene composition. Hazeincreases as more light is scattered or absorbed. Visible light is thatradiation visible to the human eye, in particular, wavelengths between400 nm to 700 nm. As disclosed herein, the haze was measured forpolystyrene composition samples with a thickness of around 10 mil orless. As is obvious, there will be more haze with thicker samples andless haze with thinner samples. Sample thickness of between 10 and 20mil is preferred because typical commercial embodiments of an orientedpolystyrene composition of the present invention are between 10 and 20mil. However, it is understood that if thicker oriented polystyrenecompositions are prepared, haze may be greater than 10%.

The impact strength of a biaxially oriented polystyrene composition ofthe present invention is between about 10 lbs to about 200 lbs attemperatures between -30° F. and ambient temperatures. As used herein,ambient temperatures are typical room temperatures, roughly between 60°F. and 100° F. The impact strength of a biaxially oriented polystyrenecomposition can be measured with commercially available testingequipment. As disclosed in greater detail in Example 2 below, the impactstrength was measured on a DYNATUP Drop Weight System (General ResearchCorporation, 5383 Hollister Avenue, Santa Barbara, Calif. 93111). Theimpact strength of a biaxially oriented polystyrene compositionincreases as the thickness of the composition is increased. The impactstrength of polystyrene compositions, with a thickness of around 15 mil,was determined to be between about 10 lbs to about 70 lbs. One ofordinary skill in the art could increase the impact strength of abiaxially oriented polystyrene composition of the present invention byincreasing the thickness of the sample. Impact strengths of greater than200 lbs for biaxially oriented polystyrene compositions that are thickerthan 30 mil are thus contemplated by the present invention. In apreferred embodiment, the polystyrene compositions disclosed herein canbe prepared by admixing general purpose polystyrene (GPPS) with highimpact polystyrene (HIPS) which includes rubber. Alternatively, GPPS canbe admixed with neat rubber.

A biaxially oriented polystyrene composition includes from about 0.1 toabout 1.4 weight percent rubber based on the total weight of thecomposition. The rubber increases the impact strength of thecomposition. In a preferred embodiment, the rubber content is betweenabout 0.15 to about 0.8 weight percent. Rubber particles with averagediameters of between 0.1 micron to 10 microns are used in a polystyrenecomposition of the present invention. Preferred rubber particles haveaverage diameters of between about 0.15 micron to about 8 microns. Mostpreferably between about a 0.2 microns to about 0.6 microns. Smallerrubber particles are preferred because the use of smaller particlesyields compositions with better optical clarity. Smaller rubberparticles scatter or absorb less incident light than do largerparticles.

The impact strength of a biaxially oriented polystyrene composition ofthe present invention is between about 10 lbs to about 200 lbs attemperatures between -30° F. and ambient temperatures.

In one embodiment, a biaxially oriented polystyrene composition consistsessentially of about 88 to 98 weight percent GPPS and about 2 to about12 weight percent HIPS. In a preferred embodiment, the biaxiallyoriented polystyrene composition consists essentially of about 92 to 98weight percent GPPS and about 2 to about 8 weight percent HIPS. Asdiscussed herein, preferred HIPS typically include rubber particles withaverage diameters of between about 0.1 micron to about 10 microns.

The present invention discloses a process for preparing a biaxiallyoriented polystyrene composition. In a preferred embodiment, thebiaxially oriented polystyrene composition is prepared by admixing GPPSand HIPS in any extruder equipment. The admixture is then subsequentlystretched biaxially to orient the polystyrenes molecules with theappropriate machine direction and transverse direction orientationequipment. (Marshall & Williams, Providence R.I.) As discussed above,biaxially oriented compositions prepared by the disclosed process have ahaze of less than about 10% and impact strength of about 10 lbs to about200 lbs at temperatures between -30° F. and ambient temperatures.Furthermore, the compositions include a rubber content of between 0.1and 1.4 weight percent based on the total weight of the composition.

Both GPPS and HIPS are available commercially. Preferred GPPS includeGPPS available from Novacor, Dow Chemical, Chevron and BASF. PreferredHIPS include but are not limited to DOW Chemical XU-70007.00, AIM 4900;Novacor 4300; BASF 5600; and Chevron Oil HG-200, HG-210, HG-100. AnyHIPS with rubber particles with average diameters between 0.1 micron and10 microns are contemplated by the present invention.

The biaxially oriented polystyrene composition can be prepared byadmixing about 88 to about 98 weight percent GPPS with about 2 to about12 weight percent HIPS. In a preferred embodiment HIPS comprises about 2to about 8 weight percent of the total weight of the composition.

The biaxially oriented polystyrene composition of the present inventioncan be formed to provide a container for transporting and storingperishable items at temperatures between -30° F. and ambienttemperatures. The composition can be formed and trimmed into containersby well known forming techniques, including the use of thermoformingmachines produced by Brown Machine--Models #C-2110R andCS2100--(Division of John Brown Plastics Machinery, 330 North RossStreet, Beaverton, Mich. 48612); Irwin International, Inc.--Models #Magnium, 341 and 342 --(1312 North 16 Avenue, Yakima, Wash. 98902); LyleIndustrial, Inc. (4144 West Lyle Road, Beaverton, Mich. 48612); andPackaging Industrial Group, Inc.,--Sentinel Model #81DF --(Hyannis,Mass. 02601).

EXAMPLE 1

A Biaxially Oriented Composition Prepared with GPPS and DOW XU-70007.00

A biaxially oriented composition comprising Novacor GPPS and DOWXU-70007.00 was prepared by admixing GPPS and XU-70007.00. The admixturewas biaxially stretched and further formed into a container about 15 milthick on thermoforming machines. Haze was measured by using Byk GardnerHaze Meter (HazeGard) (Model #XL-211 Silver Springs, Md.). All of thecompositions were clear with the haze of less than about 10 %.

The low temperature drop test was performed by filling a 15 mil thickbiaxially oriented polystyrene container with 0.5 pound of water andfreezing the water at -9° C. The container containing the ice wasdropped from 9 feet to determine the failure of the containers. Thefollowing table summarizes the results.

    ______________________________________                                        Percent HIPS                                                                  ______________________________________                                        0.5               cracked at corners                                          4.5               no cracking                                                 6.5               no cracking                                                 ______________________________________                                    

EXAMPLE 2

Biaxally Oriented Compositions Prepared with GPPS and Other CommerciallyAvailable HIPS

Biaxially oriented compositions comprising GPPS and other commerciallyavailable HIPS were prepared using a small laboratory extruder and thenwere biaxially oriented using laboratory orientation equipment (T. M.Long, Inc. --Model #14 M--(40 South Bridge Street, Somerville, N.J.08876). The tested material was a 6 to 7 mil thick film. The impactstrength was determined on a Dynatup Drop Weight System. (Model #8200Standard, General Research Corporation, 5383 Hollister Avenue, SantaBarbara, Calif. 93111). Dynatup testing is a falling drop impact test inwhich the energy required for sample failure is determined by dropping aprobe onto and through a sample. During this test, a fiber optic devicetriggers an oscilloscope just before striking the specimen. A probe witha velocity of 13.9 ft/sec (4.2 m/sec) and total kinetic energy of 90ft/lb (122 Joules) was dropped on a 4"×4" biaxially oriented polystyrenespecimen. When the probe penetrated the specimen, a completeload-time-energy history of the impact was obtained from the trace onthe oscilloscope. The total energy absorbed by the specimen wascalculated and plotted against time. Dynatup testing provides totalenergy required for failure as well as the peak energy at failure. Thefollowing table lists the impact strengths of these compositions.

    ______________________________________                                                    Maximum Load (lbs)                                                Percent HIPS  12%    8%        6%   2%                                        ______________________________________                                        Novacor 4300* 26.5   26        29   28                                        HG 210*       23.6   20        20   18.6                                      HG 200*       19.7   19        15.7 17.6                                      HG 100*       17     17        18   20                                        AIM 4900*     17.5   16        18   13.6                                      ______________________________________                                         *Novacor 4300 is available from Novacor; HG 100, HG 200, and HG 210 are       available from Chevron Chemical. AIM 4900 is available from Dow Chemical      Company.                                                                 

    ______________________________________                                        COMPARISON AT EQUIVALENT RUBBER LOADINGS                                      Average Rubber  % Total Rubber                                                                             Haze                                             Particle Size, Microns                                                                        in Film Composition                                                                        %                                                ______________________________________                                        0.23            0.2          1.3                                              0.27            0.2          2.9                                              0.37            0.2          2.1                                              3.40            0.2          2.5                                              0.27            0.6          3.8                                              0.37            0.6          2.1                                              3.40            0.6          7.6                                              0.23            1.0          2.7                                              0.27            1.2          4.8                                              0.37            1.1          3.8                                              3.40            0.9          9.6                                              ______________________________________                                    

All of the compositions and methods disclosed and claimed herein can bemade and executed without undue experimentation in light of the presentdisclosure. While the compositions and methods of this invention havebeen described in terms of preferred embodiments, it will be apparent tothose of skill in the art that variations may be applied to thecomposition, methods and in the steps or in the sequence of steps of themethod described herein without departing from the concept, spirit andscope of the invention. More specifically, it will be apparent thatcertain agents which are both chemically related may be substituted forthe agents described herein while the same or similar results would beachieved. All such similar substitutes and modifications apparent tothose skilled in the art are deemed to be within the spirit, scope andconcept of the invention as defined by the appended claims.

What is claimed:
 1. A container for storing and transporting food madeof a biaxially oriented film having less than about 10% haze when hazeis measured with a sample of the film which is less than or equal to 15mil thick and having an impact strength of about 10 lbs to about 200 lbsbetween -30° F. and ambient temperature, said biaxially oriented filmconsisting essentially of about 88 to about 98 weight percentpolystyrene and about 2 to about 12 weight percent high impactpolystyrene, said high impact polystyrene further comprising rubberparticles with an average diameter of about 0.1 micron to about 10microns, wherein said biaxially oriented film has a rubber content ofabout 0.1 to about 1.4 weight percent of the total weight of saidbiaxially oriented film.
 2. The container for storing and transportingfood of claim 1, wherein the rubber content of said biaxially orientedfilm is about 0.15 to about 0.8 weight percent of the total weight ofsaid biaxially oriented film.
 3. The container for storing andtransporting food of claim 1, wherein the rubber content of saidbiaxially oriented film is about 0.2 to about 0.6 weight percent of thetotal weight of said biaxially oriented film.
 4. The container forstoring and transporting food of claim 1, wherein the average diameterof said rubber particles is about 0.15 micron to about 8 microns.
 5. Thecontainer for storing and transporting food of claim 1, wherein theaverage diameter of said rubber particles is about 0.2 micron to about 6microns.
 6. The container for storing and transporting food of claim 1,wherein said high impact polystyrene comprises about 2 to about 8 weightpercent of the total weight of said biaxially oriented film.
 7. Thecontainer for storing and transporting food of claim 1, wherein saidhigh impact polystyrene comprises about 2 to about 6 weight percent ofthe total weight of said biaxially oriented film.